1. Field of the Invention
The present invention relates to the field of fluid shear couplings of the type used as a vehicle fan drive in which a rotor is received within a driven member and coupled thereto by a viscous working fluid flowing therebetween, and in particular to such a fluid shear coupling in which the flow of working fluid is modulated by a temperature responsive valve.
2. Description of the Related Art
A variety of fluid shear couplings, also referred to as viscous couplings, have been proposed in the prior art. Many of these couplings include a rotor which is connectable with an external drive source and is received within a housing that defines a fluid shear chamber. Improvements for such couplings have been proposed along many lines including bearing structures, fluid valving, temperature controls and torque transfer.
In vehicular applications, where the fluid shear coupling is used in association with a cooling fan, it is desirable that the coupling be "engaged" only when the ambient temperature is above some predetermined limit so as to minimize power consumption by the fan when it is not needed for cooling. Accordingly, fluid shear couplings have been provided with temperature responsive valves which regulate the flow of working fluid from a reservoir to the fluid shear chamber. That is, when the temperature is low, the valve is closed and only a minimal amount of the fluid flows to the fluid shear chamber (through a bleed hole), thereby providing minimal coupling. This is the "disengaged" condition. As the temperature rises above the predetermined actuation temperature, the valve opens and fluid quickly enters the fluid shear chamber causing maximum coupling. This is the "engaged" condition.
Prior temperature responsive valves have typically involved a fluid flow aperture located near the periphery of the coupling connecting the reservoir with the fluid shear chamber, and a closure element which covers or uncovers the aperture depending upon temperature conditions. Such valves have typically been designed such that they move between an effectively closed condition and an effectively open condition in response to a relatively small temperature change centered about the design operation temperature. That is, as the ambient temperature rises and approaches the design operation temperature, the coupling goes from a disengaged condition to an engaged condition quite abruptly, and vice versa, as the ambient temperature drops. Consequently, the coupling is in effect either maximally engaged or minimally engaged at all times.
It would be desirable to provide a fluid shear coupling with a temperature responsive valve which would modulate the degree of coupling over a relatively wide range of ambient temperature. Thus, no more power than necessary for cooling would be used by the coupling as dictated by the prevailing temperature conditions. In the preferred embodiment, the present invention provides a fluid shear coupling with an improved valve which modulates the flow of working fluid therethrough in more direct proportion to temperature than prior valves, and which provides such modulation over a relatively wide temperature range.